The right way to Select ExpressLRS Receiver: Frequency, Variety, Gemini, Compatibility and Past

As ExpressLRS continues to evolve, there at the moment are an amazing variety of receivers to select from. That’s why I created this information: to interrupt down the fundamentals of ExpressLRS receivers, clarify the distinction in frequency, antenna modes and options, and make clear compatibility with transmitters so you may determine which receiver is greatest to your wants.

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In case you’re new to ExpressLRS, try my in-depth tutorial on getting began and organising your radio hyperlink: https://oscarliang.com/setup-expresslrs-2-4ghz/

Frequency: 2.4GHz vs 900MHz

Most ExpressLRS receivers help one frequency band (both 2.4GHz or 900MHz), whereas some receivers help dual-band (each 2.4GHz and 900MHz), although they can be switched to a single frequency should you choose.

2.4GHz

• The most typical frequency in FPV radio hyperlinks
• Offers wonderful vary (tens of kilometers in superb circumstances), sufficient for many pilots
• Helps extra pilots flying on the similar time due to the broader bandwidth
• Compact antennas — superb for FPV drones the place weight and house matter

900MHz

• Covers 868MHz (EU) and 915MHz (US/remainder of the world) — you have to select the right frequency to your area to keep away from interference
• Because of the decrease frequency, it’s higher in penetration and vary in comparison with 2.4GHz, most well-liked for long-range flights
• Drawbacks: bigger antennas and added weight, which could be a problem for smaller builds

For many pilots, 2.4GHz is enough. 900MHz is highly effective, however typically overkill except you’re particularly flying lengthy vary or in difficult environments.

The right way to Determine 2.4GHz and 900MHz Receivers

You’ll be able to often inform what frequency a receiver helps by simply wanting on the antenna:

• 2.4GHz receivers → Very small antennas; solely work with 2.4GHz transmitters
• 900MHz receivers → Giant antennas; solely work with 900MHz transmitters
• Twin-band receivers → Antennas typically function each 2.4GHz and 900MHz components, permitting them to work with each transmitter sorts

2.4Ghz

900Mhz

2.4Ghz + 900Mhz

One other indicator is the RF chip used:

• Single frequency chips:
  • SX128X (SX1281) → 2.4GHz
  • SX127X (SX1276) → 900MHz
• Twin-band chip:
  • LR1121 → Helps each 2.4GHz and 900MHz

Antenna Modes

I like to consider antenna modes in two classes: Primary Mode and Gemini Mode.

Primary Mode

Primary Mode

Primary mode is how radio hyperlinks conventionally works in FPV drones. The transmitter sends one packet at a time to the receiver.

Variety

If the receiver has two antennas, it might make the most of range to enhance sign reliability.

The primary advantage of range is which you could mount antennas in several orientations or places on the drone. This helps:

• Optimize antenna alignment because the drone continuously modifications perspective in flight
• Remove sign blind spots

However there are two varieties of range, relying on the receiver {hardware}.

Antenna Variety:

• 1 RF chip + 2 antennas
• The receiver robotically chooses whichever antenna has the stronger sign at that second
• Helps cut back sign dropouts brought on by antenna orientation or multipath interference

True Variety:

• 2 RF chips, every with its personal antenna (basically two receivers on one board)
• Each antennas are continuously listening and evaluating indicators, the cleaner sign is chosen in actual time
• Provides higher vary and reliability than antenna range, however comes at the price of greater value and energy consumption

The right way to Inform the Distinction

Each antenna range and true range receivers have two antennas. The distinction is within the {hardware}:

• Antenna Variety → 1 RF chain
• True Variety → 2 RF chains

In case you examine the board intently, true range receivers may have two unbiased RF receiver chains, whereas antenna range receivers solely have one.

Gemini Mode

In case your ExpressLRS transmitter has two antennas, it’s an excellent indication that it helps Gemini Mode. Transmitters with just one antenna are restricted to Primary Mode (and variety).

Gemini Mode makes use of two transmitters, every with its personal antenna, to ship the identical information packet on barely totally different frequencies (e.g. ~40 MHz aside on 2.4GHz, or ~13 MHz aside on 900MHz). A real range receiver can choose up these two indicators independently, successfully creating two separate radio hyperlinks. If one hyperlink experiences interference or sign loss, the opposite can take over — considerably bettering hyperlink high quality and reliability.

I’ve written a full breakdown how ExpressLRS Gemini Mode works: https://oscarliang.com/expresslrs-gemini/

Observe: Gemini Mode solely works with true range receivers. In case you’re utilizing a single-antenna receiver, or one which solely helps antenna range, it would default again to Primary Mode.

Gemini Xrossband (Gem-X)

Gemini Xrossband, or Gem-X, takes Gemini Mode one step additional. As an alternative of transmitting two equivalent packets on the identical band, it sends them concurrently on each 2.4GHz and 900MHz.

This requires:

• A dual-band transmitter that helps Gem-X
• A dual-band receiver with Gem-X help

Due to this, Gem-X setups are often the costliest ExpressLRS possibility. However they provide most:

• Vary
• Redundancy
• Resistance to interference
• General hyperlink high quality and reliability

Examples of suitable {hardware} embrace the Radiomaster GX12 radio and the BetaFPV SuperX Nano Receiver.

Antenna Sorts

Most ExpressLRS receivers use an IPEX MHF1 connector (also referred to as U.FL) for attaching exterior antennas. Nevertheless, you’ll additionally discover receivers with onboard antennas, equivalent to ceramic tower antennas and flat antennas.

• Onboard antennas – Glorious for compact builds and short-range flying. They save weight and house however sacrifice efficiency.
• Exterior antennas – Advisable for higher vary and reliability.

Receiver Anatomy

RF Chip

The RF chip is among the most vital parts of a receiver. It determines what frequencies, packet charges, and modulation modes the receiver helps.

Frequent chips are:

• SX127X (e.g., SX1276) → Helps 900MHz
• SX128X (e.g., SX1281) → Helps 2.4GHz
• LR1121 → Helps each 900MHz and a pair of.4GHz

Amongst these, the LR1121 is essentially the most highly effective, supporting dual-band operation together with extra modulation modes and packet charges.

BetaFPV ELRS Lite 2.4GHz Receiver with SX-1281 Chip	BetaFPV SuperX Mono Receiver with LR1121 Chip (Helps each 900MHz and a pair of.4GHz)
Radiomaster RP4DT 2.4GHz Receiver with SX1281 Chip	BetaFPV SuperX Nano Receiver with LR1121 Chip (Helps each 900MHz and a pair of.4GHz)

Right here’s a diagram displaying what frequencies, packet charges and modulations every chipset help.

PA and LNA

PA (Energy Amplifier) and LNA (Low-Noise Amplifier) are parts that enhance the efficiency of the receiver which might be vital for vary.

• PA can enhance telemetry sign hyperlink as telemetry requires the receiver to transmit sign again to the radio.
• The LNA will increase the sensitivity of the receiver and offers you higher vary and hyperlink stability.

Observe that some compact or finances receivers don’t have PA and LNA to save lots of house and price.

TCXO

TCXO (Temperature Compensated Crystal Oscillator) is a kind of oscillator that gives a much more secure frequency with a a lot wider temperature vary. That is vital as a result of when parts warmth up or quiet down, it might trigger frequency shift within the oscillator, and if that frequency shift is outdoors of a sure margin, your receiver will lose reference to the transmitter. So, having a receiver with a TCXO will guarantee optimum efficiency and reliability.

Processor

For the typical person, you don’t really want to know the processor (or MCU) however is price mentioning. Frequent processors in ExpressLRS receivers embrace:

PWM Receivers

Some ExpressLRS receivers include PWM outputs (servo header pins). These are particularly helpful for wings or conventional RC fashions with out flight controllers, the place it is advisable plug in servos and ESC on to the receiver.

For multirotors, nonetheless, PWM receivers aren’t generally used — it’s a lot simpler and extra environment friendly to attach a receiver to the flight controller by way of a serial port (UART).

Some PWM receiver choices:

Compatibility

RX

2.4Ghz Single Antenna RX

2.4Ghz Antenna Variety RX

2.4Ghz True Variety RX

900Mhz Single Antenna RX

900Mhz Antenna Variety RX

900Mhz True Variety RX

Twin Band Single Antenna RX

Twin Band Antenna Variety RX

Twin Band True Variety RX

TX

2.4Ghz Single Antenna TX

2.4Ghz Gemini TX

900Mhz Single Antenna TX

900Mhz Gemini TX

Twin-band Single Antenna TX

Twin-band Gem-X TX

The right way to Swap Between Antenna Modes

In case you’re utilizing a Gemini-capable module (one with two antennas), such because the Nomad module or Radiomaster GX12 radio, you may change antenna modes utilizing the ExpressLRS LUA script. In case your transmitter solely has one antenna, you don’t want to fret about antenna modes, it would function in Primary Mode by default, with no switching required.

• Primary Mode: Underneath Antenna Mode, choose Ant1, Ant2 or Swap (robotically alternates between antennas).
• Gemini Mode: Underneath Antenna Mode, change to Gemini.
• Gem-X Mode: choose Gemini below Antenna Mode, and select a packet charge prefixed with “X” (e.g. X150).

In case your transmitter is twin band, you can even select which frequency to transmit on within the packet charge choices: Low band = 868/915Mhz, 2.4G = 2.4GHz.

Vital: In case your transmitter helps Gemini however you’re utilizing it with a single-antenna receiver, it is best to choose one of many antenna in Antenna Mode (e.g. ANT1). Don’t allow Gemini, for the reason that receiver can solely sync to at least one transmitter antenna — the second antenna would merely act as interference.

For extra particulars, try my full information on antenna modes: https://oscarliang.com/expresslrs-gemini/#How-to-Allow-Gemini-Mode

Conclusion: The right way to Select ExpressLRS Receivers

For five-inch quads and smaller, a 2.4GHz single-antenna receiver in Primary Mode is greater than sufficient for many freestyle and indoor pilots. In order for you further reliability for longer vary flights, contemplate upgrading to a range receiver and mount the antennas in several places on the drone to cut back blind spots.

For 7-inch or bigger long-range rigs, many pilots choose 900MHz, however as I discussed earlier, 2.4GHz is usually greater than enough. In case you already personal a 2.4GHz transmitter, I like to recommend making an attempt that first. For my part, 2.4GHz is just extra versatile. In case you actually need to discover 900MHz, then a dual-band transmitter could be the smarter selection for flexibility.

For ultra-long vary, flying in interference-heavy areas, or going behind obstacles, contemplate Gemini Mode and even Gemini Xrossband (Gem-X). The selection will depend on your wants and finances. Consider: Gemini doesn’t magically improve most vary — physics nonetheless limits how far a frequency can journey. What Gemini does present is healthier resistance to interference and redundancy for a extra sturdy hyperlink.

In case you’re shopping for a brand new transmitter module, contemplate a dual-band possibility that helps Gem-X. This manner, you’ll be suitable with just about all ExpressLRS receivers in the marketplace and be extra future-proof. With regards to receivers, although, the only option relies upon fully in your flying type and necessities.